In the prior art, various approaches have been taken for the implementation of flush-mounted antennas for various end uses. Some of these are adapted for flush mounting in air/space vehicles, even though deficient in certain important aspects, vis-a-vis the invention herein disclosed and described.
Cavity-backed antennas for the air/space vehicle application are particularly attractive because they readily afford flush mounting, consequently the pertinent prior art comprises cavity-backed configurations. Moreover, spiral antennas per se are known to have relatively broad beam characteristics normal to the plane of the spiral. Cavity backing of a spiral antenna is known to provide increased directional sensitivity and a favorable electrical combination otherwise, since the spiral elements are, in a sense folded, thereby permitting them to be electrically relatively long within a correspondingly small aperture.
Arrays of spiral antennas each with resonant cavity backing have been used for direction finding. Those spirals are spaced about the points of a compass and lie in vertical planes. Frequently no particular effort to compact such arrays is required, however, in U.S. Pat. No. 4,143,380 an arrangement is described in which the spirals are disposed about a cylindrical surface and share a common resonant chamber of annular cross-section within the cylindrical surface. The combination of the aforementioned Pat. No. 4,143,380 is of interest because of its teachings in respect to compaction of cavity-backed spiral antennas, but obviously it is not applicable for flush mounting, at the skin surface of an air/space vehicle.
U.S. Pat. No. 4,032,921 describes a cavity-backed spiral antenna which would readily be flush mounted, however the spiral is filamentary in nature and, even if well covered by a radome, is subject to damage from air friction heating.
A retro-directive, cavity-backed assembly of spiral radiators is shown in U.S. Pat. No. 3,508,269. Tunnel diodes are connected to provide discontinuities in the associated transmission line to cause reflection of signal energy toward the feed assembly. Etched circuit techniques are employed to produce the spiral elements from copper clad dielectric sheets. Essentially the same vulnerability to air friction heating as noted in connection with U.S. Pat. No. 4,032,921 can be attributed to this configuration.
U.S. Pat. No. 3,568,206 discloses a square filamentary sprial antenna within a square cavity. Significance is attached to a slot formed by the clearance between the cavity sidewalls and the perimeter of the spiral. Again the same vulnerability to air friction heating attaches to this device as aforementioned.
U.S. Pat. No. 4,015,264 depicts a cavity-backed spiral antenna, in which a plurality of resistively loaded monopoles are disposed within the cavity for broadbanding purposes, without overall size increase. Yet again, the filamentary spiral of this antenna would be in a plane parallel and close to the plane of the vehicle skin surface, with the result that it too would be very subject to air friction heating. Such heating produces very high localized temperatures and filamentary elements of any kind can thereby be subject to severe damage or even destruction.
The known prior art, including the teachings of the aforementioned U.S. patents do not provide truly advantageous structures for the purpose of the present invention.